Generally, a fluorescent tube has to be supplied at high frequency, for example, at frequencies of around 10 to 100 kHz. Besides, it has to receive particularly strong a.c. or pulsed voltages in the initial period in order to make it start. These pulses have to reach voltages of around 1,000 to 3,000 volts. Generally, to generate high voltages at high frequencies, the fluorescent tube is associated with a resonant network formed by inductances and capacitors, this network being connected to a d.c. or rectified a.c. supply via switches controlled so as to periodically excite the resonant network.
The implementation of a circuit for starting and supplying a fluorescent tube raises problems for the implementation of each of the system components.
As concerns the resonant circuit, one of the constraints is the high cost of the components, and especially the cost of capacitors which will have to withstand very high voltages and inductors which will have strong current running through them.
As concerns the switching circuit, it must, for the sake of economy, include the smallest possible number of switches and, preferably, all switches will have to be implementable on a monolithic silicon substrate. In practice, half-bridge systems are often used, because they impose smaller voltage withstanding constraints but they have the disadvantage of requiring at least two sets of monolithic switches.
As concerns the switching circuit control system, it has to be as simple as possible and have a low power consumption.
It should thus be clear that many compromises have to be made to provide an optimal start and supply system for a fluorescent tube, by reducing the number of components and thus the system cost.